[dublinben]

Compressed hydrogen would diffuse so quickly that the chance of a dangerous explosion is quite low. Fuel cell vehicles would actually be much safer than gasoline vehicles in this regard. As soon as you were in a catastrophic accident, all your hydrogen would vanish.

[tbrownaw]

As soon as you were in a catastrophic accident, all your hydrogen would vanish.

You would have a bottle of highly compressed hydrogen somewhere. The bottles can be designed so they don't explode when damaged, but it would take a non-zero amount of time for the hydrogen to escape. For however long that takes, you have a jet of escaping pure hydrogen that would make an excellent blowtorch if anything resembling a flame or spark gets too close.

[protomyth]

"You would have a bottle of highly compressed hydrogen somewhere."

http://www.fueleconomy.gov/feg/fcv_benefits.shtml shows another option for storage (Materials-based storage) that will probably be safer.